Basic refractories — The fusion (softening) point of a basic refractory is affected how by the addition of acid oxides?

Introduction:
Refractory selection often follows the acid–basic classification. Mixing incompatible chemistries typically forms low-melting eutectics, reducing service temperature. This question focuses on how acid oxides influence the fusion point of basic refractories (e.g., MgO, CaO based).

Given Data / Assumptions:

• Basic refractory matrix (e.g., magnesia, doloma).
• Acid oxides introduced (e.g., SiO2).
• Fusion point refers to softening/melting onset under heat.

Concept / Approach:
Acid–basic reactions create compounds or eutectic mixtures with lower melting points than the parent refractory. For instance, MgO with SiO2 can form forsterite or low-melting phases, degrading hot strength and lowering the effective fusion point.

Step-by-Step Solution:
Identify chemistry: basic refractory meets acid oxide → potential eutectic.Thermodynamic implication: eutectic formation lowers melting/softening temperature.Therefore, the fusion point is reduced when acid oxides contaminate or react with basic bricks.

Verification / Alternative check:
Phase diagrams (e.g., MgO–SiO2) show eutectics below the melting point of pure MgO, confirming the reduction.

Why Other Options Are Wrong:

• Increase/no effect: contradicts well-known acid–basic incompatibility.
• Always less than 1000°C: false; basic refractories have far higher fusion points unless severely contaminated.

Common Pitfalls:
Ignoring slag/refractory interactions; assuming purity despite service contamination.

Final Answer:
Reduced by the addition of acid oxides.